1. Field of the Invention
The present invention relates to a medium for optically storing information as in an optical disk. More particularly, the present invention relates to an optical information memory medium in which once-recorded information can be erased and new information can be recorded. The invention also relates to methods and apparatus for recording, erasing and reading information by using such a memory medium.
2. Description of the Related Art
Since a high memory speed and density are enabled in the optical storing of information, the optical information storing method has attracted attention as a promising information storing method. Among the conventional optical information memory mediums, there is a medium in which a metal film is irradiated with laser beams and fine holes are formed at the irradiated part to store information. In this medium, recording is possible, but the medium is limited in that erasure of the recorded information and the recording of new information are impossible. As the memory medium in which not only the optical recording of information but also erasure, and re-recording are possible, there is known a memory medium in which a film of an amorphous semiconductor such as Te.sub.81 Ge.sub.15 S.sub.2 P.sub.2 is used, and two structural states, that is, a stable high-resistance state (the so-called amorphous state where the arrangement of atoms or molecules is disturbed) and a stable low-resistance state (the so-called crystalline state where atoms or molecules are regularly arranged), are reversibly interchanged to effect the recording, erasure, and re-recording of information (see U.S. Pat. No. 3,530,441, and Japanese Examined Patent Publication (Kokoku) No. 47-26897).
However, in the above-mentioned erasable medium, since the state of the disturbed atom arrangement (amorphous state) is used as one state, retention of the information is inherently unstable. This is because the amorphous state is a metastable state leading to the crystalline state, and the amorphous state is readily changed to the crystalline state by the application of thermal energy or chemical energy, and thus stored information is easily lost. Moreover, since transition is effected between two greatly different states, that is, the amorphous and crystalline states, fatigue occurs during the repeated recording and erasure, and accordingly, the number of repetitions of recording and erasure is limited.
Investigations have been made into alloys which are similar to the subject materials of the present invention, but these materials have not been used as a memory medium in which information is recorded and erased between two stable crystalline states, since the above investigations were directed to finding a memory medium capable of assuming two states, i.e., between the amorphous and crystalline states (for example, M. Wihl, M. Cardona and J. Tauc, "RAMAN SCATTERING IN AMORPHOUS Ge and III-V COMPOUNDS, Journal of Non-Crystalline Solids 8-10 (1972), 172-178; G. Fuxi, S. Baorong and W. Hao, GLASS FORMATION OF SEVERAL SEMICONDUCTORS AND ALLOYS BY LASER IRRADIATION, Journal of Non-Crystalline Solids 56 (1983), 201-206; W. Eckenback, W. Fuhs and J. Stuke, PREPARATION AND ELECTRICAL PROPERTIES OF AMORPHOUS InSb, Journal of Non-Crystalline Solids 56 (1971), 264-275; J. Feinleib, J. deNeufville, S.C. Moss and S.R. Ovshinsky, RAPID REVERSIBLE LIGHT-INDUCED CRYSTALLIZATION OF AMORPHOUS SEMICONDUCTORS, Applied Physics Letters, Vol. 18, No. 6, Mar. 3 (1971) 254-257; IBM Thomas J. Watson Research Center, LASER WRITING AND ERASING ON CHALCOGENIDE FILMS, Journal of Applied Physics, Vol. 43, No. 11, Nov. (1972), 4688-4693).